The term "electrostatic motor" is used herein for any device having a pair of electrodes which are movable relative to each other and which are separated by a gap in such a manner as to constitute a capacitor, the electrodes of the capacitor receiving charges that develop electrostatic forces of attraction or repulsion between the electrodes.
Such electrostatic motors are used in rate gyros, for example, as precession motors for exerting a return torque on a spinning flywheel to return it to a central position. The flywheel constitutes, for example, a moving electrode connected to ground potential, with fixed electrodes being provided in the vicinity of the flywheel to employ the electrostatic effect to exert suitably directed forces thereon to constitute a return torque. The charge received by the fixed electrodes is determined as a function of a force (or torque) reference signal which is itself derived from a position signal indicative of the angle of the flywheel relative to its central position.
This type of rate gyro is described, for example, in the present Assignee's French patent application No. 83-05464. In addition to acting as precession motors, (ie. applying a return torque to a flywheel), electrostatic motors are also used as flywheel lift motors (ie. for applying an axial force to support the weight and oppose accelerations where necessary).
Heretofore, such electrostatic motors have been servo controlled by connection to a constant voltage source. Such a method of control nevertheless suffers from the drawback of developing a force whose magnitude depends on the inter-electrode gap, and thus varies over time since any movement of the moving electrode changes the gap and hence changes the capacitance of the capacitor. The force is given by the expression: ##EQU1## where:
V is the voltage applied to the electrodes,
S is the overlapping area of the electrodes, and
e is the width of the gap.
The effectiveness an electrostatic motor is thus not constant, and depends in particular of the tilt of the flywheel (in the case of a precession motor for a rate gyro). Further, it is generally not possible to directly measure the force as effectively applied, and the apparatus does not lend itself to direct measurement of the gap. Additional position detecting means must thus provided for measuring gap width.
To remedy this drawback, proposals have been made to use a constant current source, for example as in French published patent specification No. 2 241 385, which describes an electrostatic return motor for an accelerometer including a swinging mass. In that case, the measured value of the voltage between the return electrodes includes information concerning the position of the swinging means relative to its rest position.
However, the drawback of a non-linear return force remains.
Preferred embodiments of the present invention remedy these various drawbacks by providing control on the basis of constant charge instead of on the basis of constant current.
The force is then given by the expression: ##EQU2## where Q is the charge stored in the capacitor.
It will be observed that the gap width e no longer appears in the expression.